The present invention relates to a radiation source; and more particularly, to an improved apparatus and method for generating polychromatic radiation in the infra-red and near infra-red wavelengths.
There are several types of instruments that measure the different characteristics of radiant energy. For example, the intensity of radiant energy is typically measured by a radiometer. The emission and wavelengths of electromagnetic radiation are produced and measured by the well known spectrometer; and a narrow corner of the spectrum of electromagnetic radiation is typically isolated by an instrument referred to as a monochrometer.
Since one of the characteristics of radiant energy is a function of the temperature of an object, and different materials exhibit specific radiant energy characteristics at various temperatures; measuring instruments of this type are particularly useful for remote measuring and recognition. For example, these instruments are particularly useful for space applications, where it is desired to determine the temperature and radiant energy characteristics of a remote vehicle; and to recognize a specific space object as it rapidly changes temperature upon reentry into the atmosphere.
In order to be effective, however, particularly in attempting to establish the identity of a particular object in space, the signature recognition of the radiant energy must be precise. This is extremely important during reentry to the atmosphere where the space object is subjected to rapidly changing temperatures. Therefore, this type of instrument requires calibration in accordance with a reliable standard for all temperatures and materials to be measured. It is customary to calibrate radiation measuring devices for the infra-red and near infra-red regions by using a source of radiation in accordance with the "black body" theory. A black body is defined as an ideal body that reflects none of the radiation falling upon it, thus theoretically having an absorption rate of 100%. In actual practice, a radiator that emits over ninety-five percent of impinging energy, is commonly referred to as a black body; and is said to generate Planckian radiation.
When a black body is heated, the character of the radiation emitted by it, including flux density and spectral energy, varies as a function of temperature, in accordance with well known physical laws. For example, the total emission of radiant energy of a black body takes place at a rate expressed by the Stefan-Boltzman or fourth power equation, while the spectral energy distribution conforms to Planck's Equation. Thus, if the emissivity and temperature of an infra-red radiation source are known, the Planckian equation may be used to properly calculate the power and spectrum of such radiation source. In other words, an accurate calculation may be made as to how much power, and at what wavelength, such a radiation source contains. The Planckian equation may be expressed as follows: ##EQU1## where E .lambda.(T) is the monochromatic emissive power of a body at temperature T.
T=temperature of the body. PA1 .lambda.=wavelength at which the power is being calculated. PA1 C.sub.1 =calculation constant (374.15.times.10.sup.-18 Wm.sup.2) PA1 C.sub.2 =calculation constant (14.388.times.10.sup.-3 mK) PA1 .epsilon..lambda.=emissivity of the body at .lambda..
The above equation calculates the power emitted by the body for each wavelength. To calculate the total power emitted by the body in question, the function must be integrated, or ##EQU2## which is equal to .gamma.T.sup.4, the Stefan Boltzman Law.
Experimentally, a black body may be considered an almost completely closed cavity in an opaque body, such as a jug. In the laboratory, a black body is usually made from an elongated cylinder that is blackened on the inside and completely closed, except for a narrow slit in one end. Radiation closely resembling black body or Planckian radiation escapes from the slit when the cylinder is heated. It is customary to heat these enclosures electrically and to vary the temperature as you would a conventional resistance heating device.
Such a radiation source was limited as a radiation standard for the calibration of radiation measuring instruments, for example, in that it was difficult to provide a uniform temperature to the enclosure, thus hindering the generation of uniform radiant energy. Further, it was difficult to heat the enclosure to the temperatures required in space applications. Finally, it was a practical impossibility to rapidly change the temperature of the enclosure in order to provide a standard corresponding to the real time simulation of changing environmental conditions, such as atmospheric reentry. Such a radiation source required substantial mathematical interpretation and theoretical calculations for calibrating radiant energy measuring devices at high and rapidly changing temperatures up in the range of 3000 Kelvins or more.